1. Computer Science Division, University of Camerino, Camerino, Italy. E-mails: diletta.cacciagrano@unicam.it, flavio.corradini@unicam.it, rosario.culmone@unicam.it, leonardo.mostarda@unicam.it, claudia.vannucchi@unicam.it
2. Department of Computer Science, Middlesex University, London, UK. E-mails: n.gkorogiannis@mdx.ac.uk, f.raimondi@mdx.ac.uk